The present invention relates to the formulation of aqueous catalytic solutions intended to activate non-conductive surfaces and to allow subsequent electroless deposition of metals, and more particularly the formulation of solutions containing one or more noble metals capable of rendering catalytically active a non-metallic surface with respect to the solutions for the electroless deposition of metals such as copper, nickel, cobalt, and alloys thereof.
Electroless metallization is mainly practised on surfaces of non-conductive materials, such as plastics, glass and ceramic, and has a particular importance in the industry of ornamental articles and, in the electronic industry, for the manufacturing of printed circuits with through holes.
A method of activation hitherto used comprises a step of catalyzation of the non-conductive surfaces, in which the surface to be metallized is made to adsorb active catalytic nuclei which are adapted to produce, when the piece is immersed in the subsequent suitable solution, the reaction of electroless deposition of a thin conductive metal layer suitable for allowing the successive step of electrolitic deposition.
All noble metals, both of the gold group and of the platinum group, can produce activating solutions; at the present time, however, the noble metal which is mostly used in the industry as a catalyst is palladium.
By utilizing this metal, the process of activation can be carried out according to two methods: according to the first method, the articles to be metallized, whose surface has been suitably cleaned and treated, are immersed first in a solution, acidic because of the presence of hydrochloric acid, containing stannous chloride, and then, after a thorough washing, in a solution, also acidic due to the presence of hydrochloric acid, containing palladium chloride.
This method gives rise to considerable disadvantages, both because of the poor stability of the solutions and of the poor adherence between the metal deposit and the support. This latter is an extremely serious disadvantage, in particular in the production of electric through-hole printed circuits, where often the plastic support is coated with a thin copper lamina on one or both sides. In fact, in the presence of copper surfaces, the acid solution containing the noble metal deposits, by chemical displacement, a non-adherent layer of noble metal, which needs to be mechanically removed from the surface of copper laminate, thus giving rise to a considerable increase in the cost of production and rendering practically impossible the practice of automated processes.
The second method, according to the known techniques, consists in catalyzing a non-conductive or partially conductive material (as in the case of the laminates for printed circuits) in a single-step catalytic solution resulting from the mixture and the respective products of reaction between the three components which so far have been deemed to be basic for obtaining catalytic solutions which are sufficiently active and stable even after many weeks, by avoiding the formation of precipitates or crystallization; such components are hydrochloric acid, stannous chloride and palladium chloride.
Although it is possible, theoretically, to use other soluble palladium and tin salts, preference has always been given to the use of halides, such as palladium chloride and stannous chloride, since the presence of chloride ions at high concentration has always been deemed to be irreplaceable and essential. Although also other halogenic acids may be used, hydrochloric acid is preferred.
It should also be noted that not only the presence of hydrochloric acid (HCl) at high molar concentration has always been considered as fundamental for attaining active catalysts stable during their preparation, but it has always been considered as indispensable for such acid to be present also, at a high concentration, in the solutions which are subsequently diluted, to allow their industrial utilization. In fact, a low concentration of mineral halogenic acid gives rise to instability of the system, with the consequent loss of catalytic activity and precipitation or crystallization of the components of the mixture.
The resulting need of operating, even in the diluted phase, with aqueous solutions containing 20-30% by volume of hydrochloric acid, has always been the cause of very serious disadvantages as regards both the plants and equipments and, especially, the health of the operators, owing to the continuous formation, in the working rooms, of fumes which are highly corrosive and detrimental to the respiratory organs. Even if, according to recent studies of some authors, the concentration of free hydrochloric acid may be considerably reduced, in the dilution step for industrial purposes, by utilizing other sources of chloride ions (Cl.sup.-), such as for example the chlorides of the alkali metals, nevertheless, high concentrations of the mineral halogenic acid are always been deemed indispensable in the preparation of the concentrated catalyst. Moreover said technique, which has been introduced recently, has some practical disadvantages, such as an extremely high saline concentration (near to saturation point) of the industrially used solutions with a low concentration of noble metal, a lower stability of the system, with the consequent tendency to the formation of precipitates, and the poor industrial practicability of the operation of dissolving large quantities of a solid salt. Examples of the proposals mentioned hereinabove are described in U.S. Pat. No. 3,874,882 and the article by Feldstein in the review "Plating", June 1973, 60-611 (1973).